1. Field of the Invention
This invention relates to a semiconductor laser device using an optical semiconductor device, particularly, InGaAlAs.
2. Related Art
In a crystal growing method of a semiconductor laser on an InP substrate, a metal organic vapor phase epitaxy is mainly used in which the crystal composition is mainly formed of InGaAsP, and Zn is used for a p-type dopant and Si, Se is used for an n-type dopant. Si or Se as the n-type dopant less diffuses, while Zn as the p-type dopant diffuses easily. Accordingly, in a case of fabricating, for example, a structure in which Zn is doped only to a barrier layer in a multiple quantum well structure of thin film (modulation doped multiple quantum well structure), since Zn diffuses from the barrier layer to the well layer, it was difficult to attain a steep doping profile. Further, as an example other than the thin film, in a case where the concentration of Zn in a p-type guiding layer or a p-type buried layer is increased, although the laser characteristic is improved to a certain level of the Zn concentration, the laser characteristic is deteriorated abruptly since Zn diffuses as far as the undoped active layer when the Zn concentration is excessively high. The following examples are reported for the relation between the doping characteristic and the laser characteristic in the modulation doped multiple quantum well structure or the p-type guiding layer.
(1) For the Zn modulation doped multiple quantum well structure of an InGaAsP laser, there is an example of disposing InGaAs of 0.7 nm thickness in the barrier layer as a Zn diffusion suppression layer. This example is disclosed, for example, in Japanese Patent Laid-open No. 131105/1995 (Document 1).
(2) For the p-type modulation doped structure other than Zn, there is an example of the study of Be. This example is disclosed, for example, in Applied Physics Letters, p 78, vol. 51 (1987) (Document 2).
(3) On the other hand, an InGaAlAs has been noted as the crystal materials other than InGaAsP and Zn doping for the InGaAlAs system is reported. This is studied, for example, in Journal of Crystal Growth, p 463, vol. 135 (1994) (Document 3).
(4) Relation between the Zn concentration of a p-type guiding layer or a p-cladding layer and the device characteristic is also reported in the InGaAsP laser. This is disclosed, for example, in IEEE Photonics Technology Letters, p 1558, vol. 9, No. 12 (1997) (Document 4).